he relapsing fever spirochete, Borrelia hermsii, demonstrates clonal polymorphism of its major surface antigens, the pI roteins. Differential expression of one of several distinct I proteins appears to be the basis for the antigenic variation hat occurs during relapsing fever. Our current studies are ocused on the molecular mechanism of antigen switching in orreliae and, to this end, we are isolating the gene or genes or pI proteins. This project has been aided by the evelopment of several monoclonal antibodies that are either erotype-specific or cross-reactive between serotypes and by artial amino acid sequencing of two pI proteins. These tudies should not only lead to a better understanding of the athogenesis of an infectious disease that has had during its pidemics historical impacts, but may also reveal more about ene expression in procaryotic organisms.